This invention relates to a method of and apparatus for determining intraocular pressure, automatically corrected for variations in corneal thickness.
Accurately calculating intraocular pressure (IOP) is important for determining the existence of disorders of the eye, such as glaucoma, which is a leading cause of blindness throughout the world. The normal human eye has an IOP of 12-19 mmHg. A high IOP is associated with glaucoma. All of the treatments available for glaucoma at present use normalization of IOP as the goal of treatment.
Accurate measurement of IOP involves cannulation of the eye. This invasive method cannot be used in general clinical practice; thus, a number of indirect methods for estimating IOP have been devised.
Applanation tonometry is the widely used method for indirect measurement of IOP. Applanation tonometry involves application of pressure at the front of the cornea of the eye, with the pressure necessary to flatten, or applanate, the surface of the cornea then used to calculate IOP. The Goldman applanation tonometer is widely used to measure IOP by this method, and has been a standard instrument for ophthamological measurement of IOP for many years.
In the calibration of his tonometer, Goldman assumed a central corneal thickness of 0.540 mm. Although he knew theoretically that central corneal thickness would influence his readings, the equipment to measure central corneal thickness accurately was not available at that time. Moreover, the fact that the human cornea has a wide range of values for central corneal thickness was not known. It is now known that human central corneal thickness can vary from 0.414 to 0.710 mm, and that race and gender may affect central corneal thickness.
In addition, new surgical techniques to correct myopia, such as photorefractive keratotomy (PRK) and laser in situ keratomileusis (LASIK) rely on resurfacing the cornea by ablating corneal tissue. Patients undergoing this increasingly popular surgery are left with thinner corneas. Although such patients are comparatively younger, over the next few decades, as they age, interpretation of IOP readings with surgically thinned corneas using Goldman's method will be misleading.
It is also now known that with Goldman applanation tonometry, the accurate reading is obtained at a mean corneal thickness of 0.545 mm. It has been shown that for every 0.070 mm variation in central corneal thickness, IOP is over- or underestimated by 5 mmHg. As a result, through the use of Goldman's method, many pressure readings in glaucoma patients may be overestimated, and many patients with genuinely high IOP may be missed. As more data become available, an increasing number of ophthalmologists are questioning the validity of readings obtained through use of the Goldman applanation tonometer.
Recently, algorithms have been developed, using both animal and human subjects, which are able to predict mmHg change in IOP with change in central corneal thickness. If both the central corneal thickness and the applanation reading are known, the effect of corneal thickness on IOP can be negated through use of these algorithms.
The use of applanation tonometry to estimate IOP is well-known in the art, and has been described, for example, in U.S. Pat. No. 5,355,884 to Bennett, and in U.S. Pat. No. 4,987,899 to Brown. Additionally, the use of ophthalmic pachymetry to calculate central corneal thickness is well-known in the art, and has been described, for example, in U.S. Pat. No. 5,512,966 to Snook. However, although it is equally well-known that the accuracy of IOP calculated via applanation tonometry is dependent on central corneal thickness, until this time no one provided automatic modification of the applanation reading based on the determination of central corneal thickness, thereby giving an accurate measure of IOP.
Accordingly, the present invention provides a method for measuring intraocular pressure through applanation tonometry and ophthalmic pachymetry, where the applanation readings are modified automatically based on the pachymetric readings, and take into account variations from the mean in central corneal thickness.
Further, the present invention provides a method for accurately determining intraocular pressure through applanation tonometry and ophthalmic pachymetry at the low and high ends of values for central corneal thickness.
Also, the present invention provides an apparatus that measures both central corneal thickness and IOP, and automatically adjusts the IOP reading according to the variation of corneal thickness around the mean, through the use of available algorithms stored in a computer chip inside a microprocessor.